Plasma spraying devices are used for spraying various flowable materials, such as powdered materials (or simply powders), in a number of applications, including, for example, in connection with coating applications. Such devices typically comprise a cathode, an anode, and a plasma channel extending between the cathode and through the anode. During operation, a plasma-generating gas is supplied to the plasma channel. The electrical arc formed between the cathode and the anode heats the gas flowing through the plasma channel, forming a plasma flow (sometimes also called a plasma stream or plasma jet). The plasma flow exits the device through an outlet in the anode at the end of the plasma channel. Several different types of plasma spraying devices are known. These types may be characterized by the position at which a flowable material is introduced (or injected) into the plasma flow. The following discussion relates to powder spraying devices. However, a person of skill in the art will appreciate that other materials may be used for spraying.
In one type of device, the powder is introduced into the plasma flow at the anode area. In some devices of this type, the powder is introduced into the plasma flow through inlets in the anode, as disclosed in, for example, U.S. Pat. Nos. 3,145,287, 4,256,779, and 4,445,021. In other devices of this type the powder is introduced into the plasma flow by feeders located outside the plasma-generating device, as disclosed, for example, in U.S. Pat. No. 4,696,855. Typically, the powder is injected substantially perpendicular to the plasma flow.
One advantage associated with devices of this type is that when the powder is injected into the plasma flow, the plasma flow is fully developed and has certain known properties, such as temperature, velocity, energy, etc. These properties depend on, and can be controlled by, the internal geometry of the plasma channel, the nature of the gas used to generate the plasma, the pressure with which the gas is supplied, the difference in electric potential between the cathode and the anode, etc. Another advantage of supplying the powder at the anode area is that the formation of plasma flow is unaffected by the powder.
However, introducing the powder at the anode area has disadvantages. Typical powders have particles of different sizes. When such powder is injected into the plasma flow, heavier particles, which have higher kinetic energy, reach the center of the plasma stream faster than lighter particles. Therefore, the lighter particles may reach the center of the plasma flow in the relatively cold zones of the plasma flow located further away from the anode, or the lighter particles may remain on the periphery of the plasma flow never reaching its center. This creates two undesired effects. First, there is a low level of homogeneity of the powder in the flow because the heavier particles are subjected to a higher temperature for a longer period of time compared to the lighter particles. The lighter particles may not be sufficiently heated for the coating applications. Second, the distribution of the coating is not uniform, and some particles may simply miss the surface to be coated, which leads to poor material economy. In other words, the powder-sprayed coating is produced using only a portion of the supplied powder. This is particularly disadvantageous when expensive powders are used. The problem can be mitigated to some extent by using powders with particles of equal mass. However, such powders are more expensive to manufacture and using them may not be a viable alternative for all applications.
To avoid problems associated with the substantially perpendicular injection of powder in the anode area of the plasma channel, attempts have been made to provide a longitudinal powder supply channel. The powder supply channel is arranged inside the plasma channel and is surrounded by the plasma flow during operation of the device. The outlet of the powder supply channel is in the anode area of the plasma channel. This interior powder supply channel, arranged inside the plasma channel, prevents adequate heating of the plasma flow and, in general, has undesirable effects on the plasma flow properties.
A further disadvantage associated with introducing the powder at the anode is that a large amount of energy is needed to maintain the high temperature and specific power (power per unit of volume) of the plasma flow so as to obtain a highly homogeneous coating. It is believed that the cause of this problem is that the temperature and velocity distribution of the plasma flow is virtually parabolic at the outlet of the plasma channel where the powder is injected. Thus, the temperature and velocity gradient and the thermal enthalpy of the plasma flow are inversely proportional to the diameter of the plasma flow. To increase the homogeneity of the sprayed coating, it is therefore necessary to increase the diameter of the plasma flow, which in turn requires a lot of energy.
In a second type of device, the powder is supplied at the inlet of the plasma channel, at the cathode. In these devices, the electric arc heats both the plasma generating gas and the powder. The cathode area is considered to be a cold zone, which allows the powder to be introduced in the center of the plasma flow. Examples of devices of the second type are disclosed in, for example, U.S. Pat. No. 5,225,652, U.S. Pat. No. 5,332,885, and U.S. Pat. No. 5,406,046.
When plasma is generated by supplying a plasma generating gas to the plasma channel and heating the gas with an electric arc of a predetermined discharge current, only a small portion of the gas forms the center of the plasma flow where the temperature is high. The remaining gas flows closer to the walls of the plasma channel, where the temperature is lower, forming the cold layer of the plasma flow. The cold powder particles interfere with the temperature increase of the plasma in the flow, and the powder in the periphery of the flow never reaches the desired temperature. Because of this temperature distribution in the plasma flow, only a small portion of the powder, supplied at the inlet of the plasma channel, flows in the high temperature center of the plasma flow and is sufficiently heated by the electric arc. The remaining powder flows in the cold layer of the plasma flow. This causes an uneven heating of the powder, which affects the quality of the surface coating. Furthermore, there is a risk of the plasma channel being clogged by the powder, which has a detrimental effect on the conditions required for a stable plasma flow.
Increasing the transfer of mass to the central part of the channel by increasing the rate of the gas and powder flows is not a practicable alternative. When the flow of the gas and powder increases, while the current remains constant, the diameter of the electric arc decreases, which just aggravates the problem of the powder accumulating in the cold layer along the plasma channel walls. Furthermore, for those particles that end up in the center of the plasma flow, the time spent in the plasma flow decreases, because the velocity of those particles increases. Therefore, the amount of the powder in the high temperature plasma flow center cannot be increased if the current remains constant. Increasing the operating current, however, causes disadvantages associated with both the design and handling of the plasma-spraying devices.
In devices of a third type, a portion of the plasma channel is formed by intermediate electrodes electrically insulated from the anode and the cathode. The powder is introduced into the plasma flow in the portion of the plasma channel formed by the intermediate electrodes, typically between two electrodes. Thus, the powder is supplied neither at the inlet of the plasma channel nor at the outlet of the plasma channel. Examples of devices of the third type are disclosed in, for example, U.S. Pub. No. 2006/0091116A1.
The device disclosed in U.S. Pub. No. 2006/0091116A1 has two plasma channel sections. The section of the plasma channel located upstream from the powder feeder is formed by one or more intermediate electrodes and is used to create optimal conditions in the plasma flow. In particular, during operation, the plasma is heated to a temperature sufficient to melt the powder throughout the entire cross section of the plasma channel. This eliminates the problem associated with powder particles traveling in the cold layer of the flow, and reduces the risk of clogging when particles stick to the walls of the plasma channel. The section located downstream from the powder feeder is also formed by one or more intermediate electrodes and is used to achieve a high level of homogeneity and temperature of the powder particles in the flow thus obviating the problems associated with supplying the powders at the anode. By controlling the properties of the downstream section, such as its length and the number of intermediate electrodes forming the section, optimal conditions of the powder are achieved. These conditions include velocity and temperature level necessary to obtain the required adhesion, structure, and porosity in the sprayed coating for a specific combination of the power material and the coating application. However, because the velocity of the plasma flow and the powder particles that it carries is relatively low, the powder particles have low kinetic energy when they exit the device.
To achieve higher velocities of powder particles, some spraying devices use throttling portions. For example so-called cold spray or velocity spray devices pressurize a relatively cold gas carrying a powder and then use a throttling portion to accelerate the gas carrying the powder to high velocities. Such devices use the kinetic energy of the powder particles for coating. Throttling portions have been long known in the art. Briefly, they are used to convert pressure of a gas flow into velocity. Throttling portions were first used in jet engines, but now they are also used in plasma generating devices. A known variation of a throttling portion is the supersonic nozzle (also called the de Laval nozzle), which is capable of accelerating the plasma flow to supersonic speeds. U.S. application Ser. No. 11/482,582 discloses the use of the supersonic nozzle in a multi-electrode plasma generating device used for cutting, evaporating, and coagulating biological tissues. U.S. application Ser. No. 11/482,582, however, is not concerned with features of the throttling portion useful for spraying applications, such as the drop in the static pressure of the plasma flow that facilitates the injection of powders and the ability to use nanoparticles for spraying.
Plasma spraying devices that use throttling portions may fall into any of the three categories set forth above. However, because of their use of the throttling portions, they are discussed separately. U.S. Pub. No. 2006/0108332 discloses the use of a throttling portion in a plasma spraying device. In particular, this publication discloses a throttling portion which is located essentially in the end of the plasma channel closest to the cathode. During operation of this device, after the plasma generating gas is briefly heated by a cathode in the heating chamber near the cathode, the gas passes through the throttling portion. The throttling portion increases the speed of the gas, in some embodiments beyond the speed of sound, and decreases the static pressure of the gas. The powder is injected into the plasma flow after the plasma passes the throttling portion at a point in the plasma channel where the plasma reaches its maximum speed and has minimum static pressure. However, because the throttling portion is arranged essentially at the cathode end of the plasma channel, the plasma flow is heated by the electric arc only while it passes through the throttling portion. Accordingly, the plasma reaches the speed of sound while it is essentially cold. Because the speed of sound is higher at higher temperatures, the absolute speed that the plasma generating gas achieves is relatively low. Due to the relatively low speed the plasma does not achieve a high power density. Furthermore, because the powder is injected in the area of the anode in the device disclosed in U.S. Pub. No. 2006/0108332, the device exhibits limitations generally associated with the devices of the first type discussed above.
U.S. Pub. No. 2006/0037533 discloses the use of a throttling portion in a thermal spraying device. The device comprises (1) a heating module used for heating a flow of gas (or plasma, in some embodiments), (2) a forming module used to decrease the static pressure and increase the speed of the gas stream; (3) a powder feeding module that is used to inject powder into the flow; and (4) a barrel module used to carry the powder in the stream, so that the powder achieves necessary properties. The publication discloses a number of different ways of implementing a heating module. For example, in some embodiments the heating module is a combustion type heating module, which heats the gas by combusting acetylene. After the gas is heated to 3,100° C., it is passed to the forming module. After the velocity and pressure of the gas flow are transformed by the forming module, the powder is injected into the gas flow in the powder feeding module. The powder particles, carried by the gas flow, achieve properties required for a particular spraying application in the barrel module.
U.S. Pub. No. 2006/0037533 discloses another embodiment of the heating portion implemented as a multielectrode plasma torch This plasma torch has a cathode, an anode, and a plurality of intermediate electrodes. The anode and the intermediate electrodes form a plasma channel. The publication further discloses a throttling portion, distinct from the one in the forming module, located essentially in the end of the plasma channel closest to the cathode. During operation of this heating module, after the plasma generating gas is heated by the cathode in a heating chamber near the cathode, the gas passes through the throttling portion. The throttling portion accelerates the flow, in some embodiments beyond the speed of sound, and decreases the static pressure of the gas.
Some devices, such as the one disclosed in U.S. Pub. No. 2006/0091116A1 discussed above, provide for injection of different flowable materials. This feature is desirable for some plasma spraying applications.
Accordingly, presently there is a need for a plasma spraying device that overcomes the limitations of the currently known devices by maximizing the energy density of the device while enabling control of both kinetic and thermal energy of the plasma flow carrying the powder particles at the outlet of the device. In particular, there is a need for a plasma spraying device and method that generates a plasma flow having a temperature and speed that enables one or more flowable materials to be injected into the plasma flow by applying a relatively low pressure, while also enabling control of the characteristics of the plasma and the flowable materials when they exit the plasma channel.